This invention relates generally to heat pump systems and, more particularly, to a heat pump for cooling a conditioned space and a method and apparatus for improving the cyclic efficiency by increasing capacity and reducing loss of suction pressure on start up.
In a typical refrigeration cycle, the refrigerant flow to the evaporator coil in a heat pump is metered by way of an expansion device which restricts the flow of refrigerant in a controlled manner to thereby control the amount of super heat in the system. Capillary tubes and thermal expansion valves are the most common types of expansion devices used in refrigeration systems and pulsed expansion valves are commonly used to control heat pump installations for use in widely varying environmental situations.
Recent control systems for heat pumps have tended toward variable speed compressors and pulse controlled expansion valves for optimizing the system parameters for the cooling and heating requirements of the conditioned space. In a typical system in which the expansion valve is pulsed to control the flow of refrigerant to the evaporator component, the expansion valve is pulsed under the control of a proportioned integral algorithm so as to control the compressor discharge temperature to maintain it at a target temperature which maximizes the steady state system efficiency. This has worked quite effectively for steady state operation in heat pump applications, but during start up of the heat pump, the expansion valve tends to remain closed because of the slow rise of the discharge temperature as the system is starting up. This causes low suction pressure and low capacity during this initial transient start up period and considerably reduces the overall efficiency of the system.